The phenomenon affects only the surface of the plaster and appears as a grid threadlike very small and irregular. It is a typical phenomenon of the withdrawal of the mortar.
You must identify the areas where the plaster is detached from the wall, simply tap on the affected area: a muffled sound indicates the area that need to be worked.
After removing the plaster detached, apply a stucco and apply a coat of primer. Then proceed with a normal paint.